The present invention relates to a variable soft-focus lens realizing a desirable soft-focus effect by changing the air space or spaces in the lens system so as to change the aberration amount in the total lens system.
Several photographing lenses or patents describing them with variable air space or spaces for obtaining the soft-focus effect are known. The recent example is U.S. Pat. No. 4,124,276.
The first condition required for a soft-focus lens is that, in the soft-focus photographing state, only the spherical aberration out of the third-order aberrations changes substantially, while the variation of the astigmatism, the coma and the chromatic aberration are small and further the soft-focus effect can be obtained evenly all over the whole picture plane. The reason is that no matter how small the variation of aberrations other than the spherical aberration is, if the amount of the spherical aberration is short, only the pictures such as are taken with a lens whose aberrations are improperly corrected are obtained, which do not produce a nice visual effect. Further, even if the amount of spherical aberration is enough, when the spherical aberration takes place partially an attractive picture cannot be obtained.
However, contrary to a lens which generates a constant amount of spherical abberation, a lens which is able to change the amount of spherical aberration and to extinguish most of the aberration is very advantageous for focusing. The reason is that it is remarkably difficult to carry out the focusing by looking at the image formed on the focusing screen when the spherical aberration is sufficient enough to obtain a fine soft-focus effect, while it is possible to increase the focusing accuracy if the focus effect is eliminated. Consequently, the better the aberrations are compensated when the focus effect is eliminated, the better the focusing can be carried out. Further, if a picture can be taken in a well corrected state, the lens is quite in a position to carry out the photographing without the soft-focus effect, which is profitable.
However, it is a contradictory demand that when the spherical aberration is increased, the change of other aberrations are checked, while all the aberrations are well compensated in the ordinary photographing state.